Mole guidance system

ABSTRACT

A subterranean missile is equipped with three mutually orthogonal coils in its body and a fourth suitably fixed in an articulatable steering member. The missile is guided along any desired underground trajectory defined with reference to a coordinate system including the plane of a dipole-quadrupole antenna system laid on the ground above. Voltages induced in the noncenterline body coils are used in a closed feedback loop to maintain the resultant magnetic field circularly polarized at the mole location. Heading errors are then revealed as a voltage phasor on the centerline coil. Rotation of the steering member sufficiently to bring the voltage induced in its coil into phase with the centerline coil voltage also aligns the steering member such that its articulation will reduce the heading error to zero. Any new trajectory may be defined as a suitable new voltage added to the centerline coil voltage.

[4 1 Jan. 23, 1973 MOLE GUIDANCE SYSTEM [75] Inventor: James ChristopherCoy ne, New

Providence, N .J

[73] Assignee: Bell Telephone Laboratories, Inc., Murray Hill, BerkeleyHeights, NJ.

[22] Filed: June 28, 1971 [21] Appl. No.: 157,570

Related US. Application Data [63] Continuation of Ser. No. 792,893, Dec.27, 1968.

[52] US. Cl. ..l75/26, 175/45, 175/61 [51] Int. Cl ..E2lb 47/022, E2lb47/024 [58] Field of Search ..l75/26, 45; 61/727; 340/32;

[56] References Cited UNITED STATES PATENTS 3,461,979 8/1969 Newfarmer..l75/56 X 3,525,405 8/1970 Coyne et al ..175/45 X 3,529,682 9/1970Coyne et a1 ..340/32 X 3,544,957 12/1970 Smetanin et al. ....l75/45 X3,589,454 6/1971 Coyne ..175/26 T0 PROCESSING CIRCUITRY FOREIGN PATENTSOR APPLICATIONS 1,240,830 7/1971 Great Britain ..l75/45 PrimaryExaminer-Samuel Feinberg Assistant Examiner-James M. l-lanley AttorneyR.J. Guenther et a1.

[57] ABSTRACT A subterranean missile is equipped with three mutuallyorthogonal coils in its body and a fourth suitably fixed in anarticulatable steering member. The missile is guided along any desiredunderground trajectory defined with reference to a coordinate systemincluding the plane of a dipole-quadrupole antenna system laid on theground above. Voltages induced in the noncenterline body coils are usedin a closed feedback loop to maintain the resultant magnetic fieldcircularly polarized at the mole location. Heading errors are thenrevealed as a voltage phasor on the centerline coil. Rotation of thesteering member sufficiently to bring the voltage induced in its coilinto phase with the centerline coil voltage also aligns the steeringmember such that its articulation will reduce the heading error to zero.Any new trajectory may be defined as a suitable new voltage added to thecenterline coil voltage.

1 Claim, 13 Drawing Figures PATENTEDJAH 23 I975 SHEET 1 0F 6 wvz/vron J.C. COVNE an (Qua ATTORNEY Pmmaumsma 3.712.391

SHEET 2 OF 6 ARTICULATION DIRECTION OF FIG. 2

PATENTEDJAN23 I975 3,712,391

SHEET 5 BF 6- F/G..5A I /6.56 (ToP vIEw FROM (ToP vIEw) ANTENNA PLANE)XQ,Y0,Z x TARGET POINT o, o, o I

DESIRED I DESIRED TRAJECTORY TRAJECTORY/ I e X|"XQ e,

PRESENT l I POSITION l,Yh l

FIG. 58 FIG. 50 (sIoE ELEVATION TAKEN (SIDE ELEVATION) IN FLUX PLANE) vv MOLE GUIDANCE SYSTEM CROSS REFERENCE TO RELATED APPLICATION Thisapplication is a continuation of my copending application, Ser. No.792,893, filed Dec. 27, 1968.

This invention relates to subterranean missiles, and specificallyconcerns a guidance system for such devices.

BACKGROUND OF THE INVENTION In the US. Pat. No. 3,529,682 of J. C. Coyneet al. there is described a system for continuously detecting theposition and attitude of a subterranean missile or Mole with respect toa fixed coordinate system relative to a pair of antennas laid on theground over the desired trajectory. In that system, the positioncoordinates of the mole in the magnetic flux plane of the antennas aswell as its pitch, yaw, and roll angles all are continuously calculatedfrom voltages detected in a trio of mutually orthogonal magnetometersfixed in the mole body. From these voltages appropriate steeringinstructions are given the mole to cause it to assume the desiredheading.

This earlier system recognized the advantages of a dipole-quadrupoleantenna. For example, calculation of the mole depth is a simple functionof the antenna half spacing. Mole distance from the center wire,measured in multiples of the antenna half spacing, is equal to the ratioof dipole to quadrupole field strength, as measured by magnetometers inthe mole at the mole location. Also, the polar angle at the origin isexactly equal to the included angle between the field vectors, asmeasured by the mole magnetometers.

While thus constituting a fully workable location detection scheme, theearlier system nevertheless has several drawbacks. For one, thecomputations of position and heading data requires extensive logiccircuitry. More importantly, the system does not provide for the mole toseek out a prescribed trajectory. Moreover, the trajectory itself is notsuperimposable onto the system.

Accordingly, the principal object of this invention is to simplify thecontrol of a subterranean missile in its movement from one point toanother along a prescribed path.

Another object of the invention is to automatically guide a mole to aspecified terminal point within a reference frame defined by a pair ofmagnetic fieldgenerating antennas.

A further object of the invention is to achieve an automatic moleguidance system in which the terminal point which the mole seeks can berespecified-at will.

SUMMARY OF THE INVENTION The present invention, broadly, is grounded inthe recognition of the advantages of a rotating magnetic field vector atthe mole location. Mole heading errors can be directly detected by phasecomparison of the induced voltages in an axial magnetometer in the molebody with that in a magnetometer mounted in an articulatable steeringsurface.

The heading error is the angle between the mole longitudinal axis and aline parallel to the magnetic field propagating antenna. The headingerror is zero when the mole axis is parallel to the antenna wire. Inthis position, no voltage is induced in the axial magnetometer; but withheading error from any mole orientation whatsoever, a small butdetectable voltage is induced in the axial magnetometer. The trajectorywhich would reduce the heading error to zero lies in the plane definedby the mole axis and a line through the mole parallel to the antennawire. Thus, a steering surface articulated in that plane will cause theheading error to reduce to zero. The steering surface is placed in thatplane by manipulating the surface until, pursuant to the invention, theaforementioned induced voltages are in phase with each other.

The rotating magnetic field can be produced by any convenient antennaconfiguration energized to produce a rotating vector. An antennaconfiguration which is preferred because of the simple geometricrelationships which it provides, is the dipole-quadrupole scheme of theaforementioned US. Pat. No. 3,529,682. For that antenna configuration,as well as for certain others, a preferred antenna energization is withsignal currents of the same frequency but differing by a phase factor.

For heading error correction pursuant to the invention, the magneticfield vector must merely rotate. It need not describe a circular pathand, in fact, can be widely elliptical with no performance penalty.

However, pursuant to a further and major facet of the invention, a fieldmaintained in circular polarization at the mole location permits controlof the moles position in three-dimensional space through the expedientof heading control. A mole trajectory originating from its presentposition and extending to a desired terminal point is specifiedaccording to this aspect of the invention by summing an appropriatevoltage phasor with the output signal of the mole axial magnetometer.The specified trajectory can be one parallel to the antenna, ordiverging or converging with respect thereto. The superimposed voltagephasor plus the induced voltage in the axial magnetometer produces avoltage phasor output identical to the phasor that would have beeninduced had the antenna been actually moved to a position directly aboveand parallel to the chosen trajectory. Steering then proceeds in exactlythe same manner as above. The steering surface is manipulated untilvoltages in its magnetometer and in the mole axial magnetometer are inphase; and the surface is articulated so as to keep them in phase. Theheading error to be reduced to zero now, however, is with respect to theline between the moles present position and the desired terminal point.

One feature of the invention, therefore, is the use of a rotatingmagnetic vector which provides a phase comparison way to find thecorrect position of a mole steering surface to counteract any headingerror.

A further feature of the invention involves defining new moletrajectories which the mole seeks out by heading control using the samephase comparison methods, thereby providing the capability of steeringtoward any desired terminal point.

The invention, its further objects, features and advantages will befully apprehended from a reading of the description to follow of anillustrative embodiment.

DESCRIPTION OF THE DRAWING FIG. 1 is a schematic perspective diagram ofthe mole and antenna system;

FIG. 1A is a perspective diagram depicting certain geometric relations;

FIG. 2 is a schematic perspective diagram of the mole articulating tail;

FIGS. 3, 3A, 3B, and 3C are schematic diagrams, taken with the plane offlux lyinG in the paper, and showing the mole and tail in variousworking positions with resulting voltages induced in certain coils;

FIG. 4 is a diagram, taken in the flux plane of the antenna system andshowing certain critical geometric relationships between the mole and amagnetic field vector;

FIGS. 5A, 5B, 5C, and 5D are diagrams illustrating the criticalgeometric relationships of mole heading in terms of a new desiredtrajectory; and

FIG. 6 is a block diagram of an overall mole guidance system pursuant tothe invention.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT The mole workingenvironment is shown schematically in FIG. 1. Current-carryingconductors in the form of a dipole antenna 10 and a quadrupole antenna11 are laid on the ground over the desired route. Their construction andmounting details are described in the aforementioned U.S. Pat. No.3,529,682. AC voltages are applied to antennas 10, 11 by poweramplifiers 12, 13, respectively. The voltages are maintained at the samefrequency, for example, 5 kHz, but, importantly, differ in phase in amanner to be described.

FIG. 2 shows a mole 14 equipped with suitable components for use in thepresent guidance system. Mole 14 consists of a body 15 and a tailassembly 16. Assembly 16 is mounted for rotational movement a full 360about the centerline axis 17 of body 15. The tail itself, designated 18,is pivotable about a fixed clevis pin 19 which is perpendicular to axis17. Control of rotation of assembly 16 and articulation of tail 18 areaffected by conventional servomechanisms consisting of rotary actuator20 and articulation actuator 50 which are shown in FIG. 6 schematically.

FIGS. 1 and 2 show three magnetometers such as induction coils 21, 22,23, fixedly mounted in mole 14. Their sensitive axes A, B, C aremutually orthogonal. As installed, axis A of coil 21 is coincident withthe mole centerline axis 17. A fourth magnetometer such as coil 24 ismounted in tail 18 with its sensitive axis D normal to the extendedcenter line axis 17 and also normal to clevis pin 19. Electricalconnections run from each magnetometer to the processing circuitry whichwill be fully explained with reference to FIG. 6.

CIRCULAR POLARIZATION As two distinct magnetic fields exist in the soilbulk, the net field illustrated in FIG. 4 at a specified point and timeis given by the vectorial addition of the separate fields, the latterdenoted D, Q. Consider their veetorial addition for an arbitrarilychosen position of mole 14 as seen in thg fl ux plane. The field vectorsD, 0 have peak values of D, Q, respectively. As the fields are out ofphase, the peaks occur at different ti m e s. At the given time, theirinstantaneous values are d, q. In FIG. 4, vector d was arbitrarily takenat its maximum value D. The vector resultant of the instantaneousvectorsz, Z, is b:

It is seen thatb rotates in space with passing time, and in generaldescribes an ellipse. Resultant vector b passes through the sensitiveaxes, A, B, C, D of coils 21-24; and voltages are induced in each coil.When the component of b along a coil is maximum, the induced voltage inthat coil is a maximum. These sinusoidal voltages are denoted V V,, V,,V respectively for the coils 21-24; voltages V, and V are schematicallydepicted in FIGS. 3A and 38.

It may be desired, as for certain purposes of the present invention, toprovide a circularly polarized magnetic field, i.e., to cause b torotate with constant angular velocity and constant magnitude. Thefollow; ing cgnsiderations then apply. FIG. 4 illustrates that D and Qd;f fer II' I, direction by the geometric angle 1'. Further, d and qdiffer in phase by the angle B which is the phase difference between theapplied sinusoidal currents i and i, to antennas 10, 11. Omitting proof,the conditions for circular polarization are:

Such conditions are maintained, for example, by monitoring the voltagesinduced in coils 22, 23 whose axes B, C lie in or near the place offlux. When the field is circularly polarized, the voltages V,, V;induced by b in coils 22, 23 have equal magnitude and degrees phasedifference. This is achieved, for example, by fixing the dipole antennacurrent i and by providing means for controlling-the phase and magnitudeof the quadrupole current i so that the field at the mole location iscircularly polarized.

If now the 'mole undergoes a position change, the field becomes slightlyelliptical, and the output voltages V,, V, of magnetometers 22, 23 willchange slightly in both amplitude and phase. A small correction of bothmagnitude Al and phase AB of i, is required to reestablish the circularfield and is achieved in the antenna current regulator 60 shown in FIG.6.

It should be understood that the invention broadly involves the use of arotating magnetic field vector. Accordingly, while the antenna signalfrequencies have in the illustrative embodiment been specified to be thesame while differing by a phase factor, other schemes exist forrealizinG a rotating vector. Also, while the dipole-quadrupole antennadescribed is applicants preferred mode, other antenna structures can beenvisioned for realizing the rotating vector. The advantages of thedipole-quadrupole configuration are that. with the driving currentdescribed earlier and circular polarization, a simple determination ofmole position is afforded. That is, the ratio of quadrupole antenna peakcurrent to dipole antenna peak current equals the distance from theantenna center wire Ila (measured in multiples 0F the antenna halfspacing). Also, the supplement of the phase difference between dipoleand quadrupole antenna currents equals the polar angle. This point andothers will now be expanded.

AUTOMATIC l-IEADING CONTROL The rotating magnetic vector b, pursuant toa major facet of the invention, enables finding the correct position ofa steering control surface such as tail 18 to counteract any headingerror.

The coordinate system notations used below and alluded to elsewhere alsobear summarization at this point. The XYZ system is with reference tothe stationary antenna as seen in FIGS. 1 and 1A, for example; theX-axis is coincident with the center wire 11a, and the XY plane isdefined by the antenna wires 10, 11, 11a. Position in the soil bulk ismeasured conveniently with reference to the XYZ system. The X'YZ' systemis a coordinate system located with reference to the XYZ system, andcomes into play when a new trajectory is to be specified in a manner tobe described. The X"Y"Z" system moves with the mole, its axes beingalways parallel to the respective axes of the XYZ system.

Heading error is any orientation of mole 14 which brings the centerlineaxis 17, and the coincident magnetometer axis A, out of parallelism withthe X" axis, as illustrated in FIG. 1A. In the earlier J. C. Coyne etal. U.S. Pat. No. 3,529,682, heading error included a pitch component (Iand a yaw component 0. Ascertaining these angles required substantialadded equipment complexity. As will shortly be seen however, the presentinvention renders it completely unnecessary, in correcting for headingerror, to calculate the values of either; not is it necessary to computea roll angle.

In FlGS. 1 and 3 the mole 14 is depicted as being on trajectory, movingtoward a target point X Y Z with zero heading angle. Thus, mole 14 isdirectly under the antenna center wire 11a. Rarely in practice, however,is the mole ideally positioned.

FIG. 1A illustrates the more usual situation. The mole is shown asdeviating from the desired heading, namely the X axis. This headingdeviation is measured by the various angles shown, and now defined.

1 called the pitch angle, is the angle between the mole axis 17 and itsprojection on the X"Y plane.

0, called the yaw angle, is the angle between the projection of the moleaxis 17 on the X"Y" plane and the X" axis.

H, called the heading angle, is the angle between the moles axis 17 andthe X axis.

M is the angle between the projection of the moles axis 17 on the Y"Z"plane and the negative Z" axis.

0 and 4) are related to H and M by the following equations:

tan M 0/ l Accordingly, FIG. 3A depicts the mole as deviating inposition and also from the desired heading by an arbitrary heading errorangle H which, it will be understood, consists of a pitch and a yawcomponent. The object is to find a rotational position for tail 18 suchthat, with articulation, the mole follows a trajectory that not onlyreduces H, but reduces H to zero.

Consider that when the mole tail 18 is articulated, the mole trajectorylies in a plane defined by the mole centerline axis and the axis of tail18. The heading error angle H can go to zero only if said plane alsoincludes the X" axis. Accordingly, the problem reduces to articulatingtail 18 in the plane defined by the X" axis and the mole centerline axis17.

It can be seen by reference to FIGS. 3A-3C that the correct rotationalposition of tail 18 occurs when the voltage induced byb in coils 21 and24 are in phase. With any heading error, axis A of coil 21 has aprojectable component in the plane of flux, as seen in FIG. 3A. In suchcase, as seen in the bottom diagram of FIG. 3A vector b induces avoltage V 4 in coil 21 which peaks when the component of vector b onaxis A is maximum. If the field were exactly circular this would occurwhen vector b and axis A were coincident. Tail coil 24 when viewed inthe flux plane has a voltage V induced by vector b.

Owing to the initial random rotation of tail 18 with respect to the molebody, however, the induced voltages in coils 21 and 24 differ by a phasefactor. This phase difference is monitored by suitable circuitry such asdepicted in FIG. 6. Rotary actuator 20 then is servoed until the phasedifference is zero as shown in FIG. 3B. Coil axes A and D now lie in aplane that includes the X"-axis. Articulation of tail 18 around pin 19at this time as depicted in FIG. 3C, will place the mole on a trajectorythat at some time is assured of bringing the mole parallel to theX-axis, thus reducing the heading error angle H to zero.

When the tail 18 articulates, the magnitude of voltages induced in coil24 will decrease, but the phase will remain unchanged. Further, as themole l4 proceeds along its curved trajectory in seeking a zero headingangle error, the phase of voltage V, will continue to remain unchangedprovided no disturbances roll the missile or divert it out of thedesired trajectory plane.

If the mole is diverted, however, then the voltage phasors V and V,,will be once again out of phase, and the inventive heading correctionprocedure will contlnuously correct. If the articulation angle is toolarge that the rotary unit 20 cannot turn the tail against soil forces,the tail is straightened and then rotated until V and V are in phase;and then the tail is rearticulated. If the articulation angle is notgreat, a new rotary position of tail assembly 16 can be sought withoutstraightening the tail.

The articulation angle itself is chosen to the desired turning curvatureand does not affect the phase matching of voltages V, and V The changein magnitude of voltage V as tail 18 articulates does provide a means ofmeasuring the articulation angle, however.

The voltages V and V are depicted in FIGS. 3A-3C as being equal inmagnitude merely for ease of illustration. They need not be; and inpractice rarely are.

It should be understood that the critical inventive point thus far isthe correction of heading angle errors by phase comparison techniques,in which the phase of the voltage induced in the axial magnetometer 21by a rotating magnetic field vector is compared to the phase of thevoltage induced in the magnetometer 24 which is fixed in the steeringmember in a predetermined orientation. This orientation is with respectto the sensitive axis D and the corresponding steering attitude whichthe steering member exhibits-or as in the instant embodiment, wouldexhibit, if articulated. Accordingly it is clear that the phasecomparison does not require that there be zero phase difference betweenthe induced voltages. It requires only that thephase difference be knownwhich corresponds to the orientation of the steering member magnetometeraxis D (and thus also the steering member itself) that will reduce theheading angle error to zero. To this end, the choice of a zero phasedifference is convenient.

HEADING CONTROL PLUS POSITIONAL CONTROL In the system so far described,control of the moles heading angle enables the moles centerline axis 17to be kept parallel to the antenna center wire 11a. In many fieldsituations, heading control in the above terms with respect to thestationary center wire would suffice as a complete guidance system if,among other considerations, the mole is originally planted parallel tothe antenna center wire and if further the trajectory is not long. Thereare, however, an infinite number of lines parallel to the center wire,any one of which could be assumed by the mole centerline axis to satisfythe requirement of zero heading angle error. Accordingly, the likelypositional error at the terminal point increases with increasingtrajectory length until it becomes intolerably large.

Pursuant therefore to a further major aspect of the invention,positional control is incorporated into the guidance scheme by causingthe system to guideagain, by heading control-along any new chosenstraightline path from the mole, and not just a path parallel to theantenna center wire. The new path advantageously can be one which linksthe mole and any desired target point.

Such target point normally is directly beneath the center wire andrepresents the terminal point of mole travel. The target point can,however, be both horizontally and vertically removed from the nominalterminal point directly below the center wire end. Further, the targetpoint can be respecified any number of times.

Implementation of position control is depicted in FIG. 6; and involvesgenerating a sinusoidal voltage, denoted R, cos (wt I The latter voltagehas a magnitude and phase equal to the polar coordinates R 1', (FIG. B)of the target point X,,Y,Z,, (FIG. 5A). The quantities R, and I, areset, either manually or otherwise, into a phase and magnitude control65.

Control 65 derives another input directly from the quadrupole antenna11. Antenna current regulator 60, as already described, monitors thevoltages V, and V, induced in coils 22, 23 and adjusts the antennacurrent i to values that maintain circular polarization.

The sinusoid R, cos (wt I is then added in summing amplifier 70 to thesinusoidal quadrupole antenna voltage R cos (011+ B). The gain ofamplifier 70 is proportional to l/X X,,, the forward distance of thetarget point.

The summing amplifier 70 output is phase-shifted in circuit 71 by thefactor 3(1r/2 B) to place the zero phase reference on the Z, axis; andthen added in summing amplifier 73 to the voltage induced inmagnetometer 21. The latter has been normalized in circuit 72 by thefield strength as measured by the rms voltage V,. Normalization can aswell be achieved with the rms voltage V, in the same fashion. The outputof the normalizing operation is a sinusoidal voltage whose amplitude isthe heading angle H measured in radians, and whose phase in the angle Mas seen in FIGS. 1A and 5B.

The output of summing amplifier 73 yields a new voltage phasor, denotedH cos (not M) whose amplitude H is the heading angle of the mole withrespect to the new desired trajectory X seen in FIGS. SA-SC;

and whose phase is the angle M shown in FIG. 5D.

The output of summing amplifier 73 is fed to two comparators 74 and 75.In comparator 74 the phase of the voltage in coil 24 is compared withthe phase of output of summing amplifier 73. The difference error signalAM is used to drive the rotary actuator 20 until the error signal goesto zero. This operation places the tail 18 in a position such that whenarticulated the heading angle with respect to the X axis will go tozero.

In the second comparator 75 the amplitude of the output of summingamplifier 73 is compared with two reference voltages, H max. and H min.These are, for example, set in manually and represent the dead zone ofthe on-off control of the tail articulation actuator 50. When I-Iexceeds H max. the valve motor is turned on, opening valve 81 tohydraulic supply pressure which drives actuator 50 through suitablehydraulic connections 82. With tail l8 articulated, the mole assumes atrajectory that begins to reduce the heading angle I-I' toward zero.When H drops below II min., comparator 75 signals valve motor 80 toremove supply pressure; and the fluid pressure is relieved as throughreturn line 83. The mole tail straightens out in about a tail length oftravel.

The capability of specifying and respecifying the mole target point hasnumerous highly useful ramifications. It is, for example, possible tocorrect for the natural tendency of the mole to surface due to thegreater strength of soil below the mole bottom, simply by moving thetarget point closer in toward the mole. Obstructions such as rocks,drain pipes and the like, whose locations are known, are readily avoidedby specifying a series of safe trajectories. It further is possible tosteer straight paths under bushes, for example, where laying the antennain a straight path is precluded. Also, the antenna does not require anyrelocation in the process of specifying the trajectory. Additionally,the mole can be caused to fare into a desired trajectory in a sweepingare, simply by leaving the gain of amplifier 70 constant. Aiming at afixed point in contrast requires changing of the amplifier gain.

Numerous additional advantages,-variations and embodiments, of theinvention will be readily envisioned by persons skilled in the art. Thespirit of the invention is limited only by the scope of the claims tofollow.

What is claimed is:

1. Apparatus for controlling heading of a steerable mole comprising,

antenna means for generating a circularly polarized magnetic vector atthe mole location in the soil;

means for specifying a target point as a first sinusoidal voltage bypolar coordinates in the plane of said field measured with respect tothe center line of said antenna means, and a lookahead distance measuredbetween the molepresent position and said target point as projected on aline parallel to said antenna means;

means for ascertaining the moles present position in terms of a secondsinusoidal voltage whose magnitude and phase correspond to polarcoordinates measured in the plane of said circular field;

means for combining said first and second voltages into a resultantvoltage representing the positional error of said mole as measured insaid circular field plane;

9 10 means for multiplying said resultant voltage by a numeans fordefining a phase difference between said merical scale factordeterminative of how fast the third voltage and the phase of a voltageinduced in mole will return to course, said scale factor being a secondmagnetometer axially perpendicular to equal to said look-ahead distance;said mole axis and disposed in the plane of the means for combining saidmultiplied resultant volt- Steering effort;

age i the voltage induced by said fi ld in an ia controlled steering ofsaid mole at this stage causally mounted magnetometer, thereby producinga '8 said Phase dlffefence to ppf a Specified third voltage whosemagnitude and phase value, and also causing the magnitude of said thirdrespond to the heading of said mole with respect to Voltage to approachzeroa line between the mole and said target point;

1. Apparatus for controlling heading of a steerable mole comprising,antenna means for generating a circularly polarized magnetic vector atthe mole location in the soil; means for specifying a target point as afirst sinusoidal voltage by polar coordinates in the plane of said fieldmeasured witH respect to the center line of said antenna means, and alook-ahead distance measured between the mole''present position and saidtarget point as projected on a line parallel to said antenna means;means for ascertaining the mole''s present position in terms of a secondsinusoidal voltage whose magnitude and phase correspond to polarcoordinates measured in the plane of said circular field; means forcombining said first and second voltages into a resultant voltagerepresenting the positional error of said mole as measured in saidcircular field plane; means for multiplying said resultant voltage by anumerical scale factor determinative of how fast the mole will return tocourse, said scale factor being equal to said look-ahead distance; meansfor combining said multiplied resultant voltage with the voltage inducedby said field in an axially mounted magnetometer, thereby producing athird voltage whose magnitude and phase correspond to the heading ofsaid mole with respect to a line between the mole and said target point;means for defining a phase difference between said third voltage and thephase of a voltage induced in a second magnetometer axiallyperpendicular to said mole axis and disposed in the plane of thesteering effort; a controlled steering of said mole at this stagecausing said phase difference to approach a specified value, and alsocausing the magnitude of said third voltage to approach zero.